As can be seen, I'm trying to show the $node->content['field_XXX'] variables (which, as the 'body variables' link states, are available) but nothing is printed. But if I replace them with something like $node->field_XXX[0]['view'], this value is shown.

If I do that, it's work : $node->field_custom[$node->language][0]['value'] = "foo";

But if I want to change my field name or to add a field without change my module code, I think it can be usefull to use $node->content
But those 2 solutions don't work : $node->content['field_custom'][$node->language][0]['value'] = "foo"; $node->content['fieldgroup_tabs']['group_desc']['field_custom'][$node->language][0]['value'] = "foo";